Ultralow frictional healing explains recurring slow slip events

Abstract

Plate motion on shallow subduction megathrusts is accommodated by a spectrum of tectonic slip modes. However, the frictional properties and conditions that sustain these diverse slip behaviors remain enigmatic. Frictional healing is one such property, which describes the degree of fault restrengthening between earthquakes. We show that the frictional healing rate of materials entrained along the megathrust at the northern Hikurangi margin, which hosts well-characterized recurring shallow slow slip events (SSEs), is nearly zero (<0.0001 per decade). These low healing rates provide a mechanism for the low stress drops (<50 kilopascals) and short recurrence times (1 to 2 years) characteristic of shallow SSEs at Hikurangi and other subduction margins. We suggest that near-zero frictional healing rates, associated with weak phyllosilicates that are common in subduction zones, may promote frequent, small-stress-drop, slow ruptures near the trench.

Silently not able to heal

Faults on the Earth rupture over time but do not always produce earthquakes. Such aseismic, or slow slip, events are an important way to release stress. Shreedharan et al . determined the frictional healing properties of a slow slip portion of the Hikurangi fault in New Zealand. They found that the ability of the material to strengthen after failure was limited, unlike for earthquake-producing events. These observations could explain why these shallow, slow slip events happen frequently and at low stress. —BG

Abstract

Frictionally weak faults that are unable to restrengthen may host frequent slow-slip events with small drops in stress.